678 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



The second well marked graptolite zone of the Ouachita shale occurs 

 at some as yet not exactly determined horizon in the upper half of the 

 formation. The best collection was made at a locality about 13 miles 

 west of Little Eock. It includes the following species : 



1. Didymograptus euodus Lapworth. 9. T. similis var. nov. 



2. D. caduceus (Salter) Rued. 10. Phyllograptus typus var. nov. 



3. D. caduceus nanna Rued. 11. Diplograptus sp. undet. (cf. D. 



4. D. sp. nov. (near D. forcipiformis, calcaratus priscus E. and W.). . 



but coarser). 12. Mesograptus sp. undet. 



5. Tetragraptus quadribrachiatus 13. Cryptograptus antennarius Hall. 



Hall. 14. C. tricornis ? Carr. 



(). T. amii ? E. and W. 15. Glossograptus hystrix Rued. 



7. T. pendens E. and W. 16. Retiograptus tentaculatus ? Hall. 



8. T. clarkei var nov. (larger). 17. Caryocaris wrighti Salter. 



Of these numbers 5 to 9 inclusive are found also in the lower zone, but 

 in each case the later occurrence is a distinguishable mutation. The 

 same species, also numbers 2 and 10, occur in the Didymograptus hifidus 

 zone in New York and Canada, while numbers 2, 5, 6, 7, and 9 are found , 

 in the Skiddaw slates of Great Britain, Numbers 3, 4, 5, 13, 15, and 

 16 — that is, most of the Axonophora — are found in and for the most 

 part confined to the Diplograptus dentatus zone in Euedemann's Deepkill 

 section. However, on account of the strong development of Tetragraptus 

 and the presence of Didymograptus caduceus in this Arkansas fauna I 

 regard it as older than the Diplograptus dentatus zone and as approxi- 

 mately equivalent to the Ashhill Quarry zone, which Euedemann places 

 as a "transitional subzone" between the Didymograptus hifidus and the 

 Diplograptus dentatus zones. 



Age of the Dictyonema flabelliforme and Olenus zones. — A point 

 on which I differ more or less from American authors concerns the age 

 of the Dictyonema flabelliforme zone, which is usually given as upper 

 Cambrian, but which I regard as not only post-Cambrian, but also post- 

 Ozarkian — in other words, as Canadian. A welcome tendency recently 

 manifested among the best Swedish stratigraphers is to place this zone 

 above the Cambrian. My proposed arrangement of the D. flabelliforme 

 zone, therefore, accords essentially with theirs. The differences in our 

 Tespective views arise from the fact that I intercalate an intermediate 

 system — the Ozarkian — between the top of their Cambrian and the base 

 of their Ordovician (or Silurian), which they begin with the Dictyonema 

 zone. That I promote the Canadian to the rank of a system and begin 

 the Ordovician with the Saint Peter in the Mississippi Valley and with 

 the first sediments of the Chazyan invasion in the Appalachian province 



